Ultrasonic transducers convert electrical alternating current (AC) signals into ultrasound, as well as converting ultrasound signals into AC signals. Ultrasonic transducers typically are piezoelectric transducers where piezoelectric crystals change size and shape when a voltage is applied. An applied AC voltage causes them to oscillate at the same frequency and produce ultrasonic sound. The beam pattern of the transducer can be determined by the active transducer area and shape, the ultrasound wavelength, and the sound velocity of the propagation medium such as air. Since piezoelectric materials generate a voltage when force is applied to them, they can also work as ultrasonic detectors. Some systems use a separate transmitter to excite the transducer and receiver to receive reflected ultrasound signal. Other systems combine both transmitter and receiver functions using a single transceiver, where the transducer is excited for a period by the transmitter and then utilized to detect reflected energy from a target after excitation.